There have been many studies in the art on therapeutic methods for cancer, such as a surgical operation, radiotherapy, chemotherapy, immunotherapy, and gene therapy. As one of them, small interfering ribonucleic acid (siRNA)-based therapy is a therapeutic method capable of silencing expression of a specific protein by bonding a siRNA to a messenger RNA (mRNA) of a RNA-induced silencing complex (RISC) and decomposing the bonding (see Non-patent Literatures 7 and 9). The siRNA-based therapy provides various solutions to genetic disorder diseases because of its repressor capability for effectively silencing expression of a target mRNA even with a small amount (see Non-patent Literatures 4 and 5). However, the siRNA is very unstable, so that it tends to decompose within a very short time inside a living organism. In addition, due to its anionic property, the siRNA does not easily penetrate through a negatively charged cell membrane and thus generates an endocytic problem (see Non-patent Literatures 7, 9, 10, and 16). In order to address the aforementioned problems, a viral delivery system has been proposed. However, this technique has some risks such as a nonspecific immune defense and some problems such as a complicated manufacturing process and is still not suitable for commercialization. Therefore, nonviral delivery systems using other types of delivery systems such as a cathionic lipid or a polymer material have been highlighted in the art. The nonviral delivery system is highly stable inside a living organism and inexpensive because of its easy manufacturing process. However, since the siRNA is small-sized and hard and has a weak anionic property, it is difficult to form a polymer complex disadvantageously (see Non-patent Literatures 10 and 13).
As means for addressing the aforementioned disadvantages of the siRNA, a nanostructure such as gold nanoparticles has been highlighted in the art. Gold nanoparticles have high biocompatibility and can be simply synthesized. In addition, it makes it possible to easily perform size control and surface modification (see Non-patent Literature 7).
If an end region of the siRNA is modified with a thiol group, the siRNA can be easily bonded to a gold nanoparticle. If the siRNA is modified and is then bonded to a gold nanoparticle, the siRNA is condensed on a surface of the gold nanoparticle, and shortcomings of the siRNA such as intrinsic hardness or a weak anionic property are eliminated, so that a polymer complex can be easily produced (see Non-patent Literatures 6 and 14).
If a cathionic polymer is used in an siRNA complex, it is possible to prevent decomposition of the siRNA inside a living organism and improve transmittance of an anionic cell membrane. A representative cathionic polymer used as an siRNA of the prior art is polyethylenimine (PH). However, it is known that cathionic polymers have cellular cytotoxicity for inducing apoptosis, and the cellular cytotoxicity increases as a molecular weight and a degree of branching of the polymer increases. Therefore, it is difficult to apply the cathionic polymer as an siRNA polymer complex for delivery to a living organism.